During the discharge of a lead acid battery, H.sub.2 SO.sub.4 is consumed and water is formed, resulting in a weakened electrolyte solution. The more deeply the battery is discharged, the lower the concentration of the H.sub.2 SO.sub.4. During discharge, insoluble lead sulfate (PbSO.sub.4) forms and adheres to both the cathode and anode in equal amounts, effectively shielding and passivating both electrodes. This "double sulfate" theory was first recorded in engineering literature over one hundred years ago. The molecular volume of the PbSO.sub.4 is appreciably greater than that of either the PbO.sub.2 or the Pb from which it is formed. The porosity of the active materials falls steadily during discharge. As discharge proceeds, the internal resistance of the cell rises due to PbSO.sub.4 formation and the decrease in electrolyte conductivity as the H.sub.2 SO.sub.4 is consumed. The reduced active material of the battery has a pronounced negative effect on the capacity of the battery.
The charging process reverses the direction of the discharging reaction. During an effective charging process, the PbSO.sub.4 is converted to Pb at one electrode and to PbO.sub.2 at the other, both in contact with the electrolyte of dilute sulfuric acid--thus restoring the materials of a fully charged cell.
However, rate of charging is important. If the concentration of H.sub.2 SO.sub.4 is very low, as it is in a battery which is sulfated, the battery will overheat during the charging process causing damage to the battery. Water of the electrolyte may be decomposed into H.sub.2 and O.sub.2 which has the potential of causing an explosion. Furthermore, the production of these gases can dislodge Pb, PbO.sub.2, or PbSO.sub.4 from the plates. These solids accumulate as a sludge at the bottom of the battery. In time, they may cause a short circuit which will render the battery useless.
Charging should also be tailored to the type of battery being charged. Deep discharge batteries with thick plates have a lower charge acceptance than other batteries. The rate at which the battery absorbs charge depends upon the diffusion of the electrolyte throughout the active material on the plate. The thicker the plate, the slower the diffusion and the lower the charge acceptance.
Gali in his U.S. Pat. Nos. 4,871,959 issued Oct. 3, 1989, 5,063,341 issued 1991, 5,084,664 issued Jan. 28, 1992 and 5,276,393 issued Jan. 4, 1994 discloses a technique for conditioning a battery during charging in which the charge includes a plurality of spikes or pulses in the charging voltage. The patents of Gali are not clear as to the dimensions of the pulses including particularly the current and voltage at the peaks of the pulses but the particular technique used by Gali in the machine manufactured in practice involves the use of a transformer which can be used to increase the voltage in a voltage pulse but in view of the increased number of windings in the secondary, this increase in voltage is obtained while reducing the current available. In practice, therefore, the pulse generated in the machine proposed by Gali, which is manufactured and sold by Motor Products and Pulse Charge Systems in USA, generates pulses of a significant voltage value but provides a current which is limited by the transformer technique to a value of the order of a few milliamps. The most recent of the above patents explains the operation of the device in terms of a resonance effect in which the pulse generates an oscillating voltage alternately increasing and decreasing the charging voltage while gradually declining toward the end of the pulse. The phenomenon of the conditioning of the battery is therefore explained in terms of the resonance of the cells.
In practice the above machine has been found to provide some conditioning of the battery thus reducing the amount of sulfation but the effect obtained is relatively slow so that a valuable effect on the state of the sulfated battery is obtained only after many days or weeks of conditioning.
U.S. Pat. No. 4,843,299 (Hutchings) issued Jun. 27, 1989 discloses a battery charging which includes a microprocessor control for detecting the current, voltage and temperature of the battery for controlling the battery charging profile. However, this patent does not disclose any techniques for conditioning the battery to reduce the state of sulfation of heavily sulfated batteries.
U.S. Pat. No. 3,816,807 (Taylor) issued Jun. 11, 1974 discloses an impedance control battery charger including a monitoring system in the form of a feedback control for automatically adjusting the DC current supplied to the battery to match the current accepting capabilities of the battery. However again the patent discloses no techniques for conditioning the battery to reduce sulfation.
U.S. Pat. No. 5,172,044 (Sasaki)issued Dec. 15, 1992 discloses a battery charger which controls the charging current and the voltage on the assumption that the battery charging follows a predetermined charging curve. Over again the patent discloses no techniques for reducing sulfation.